Page 120 - Know and Understand Centrifugal Pumps

The System Curve

The following is not very entertaining to read. The authors have included this section
so that the readers can gain an appreciation for the detailed work of the design
engineer on calculating the frictions and velocities in a piping system. Admittedly,
,. there are computer programs today that will perform these calculations in a flash.
But 30 years ago and before, these calculations were done with mental software (the
z engineer's brain), a mechanical computer (a slide rule), and a manual printer (pencil

In most cases, the design engineer and architect begin with an open field of rabbits
and weeds. Two years later there is a hotel, gasoline station, or paint factory built on
the site of the open field. And the day that the new owners open their hotel, or start
mixing paints, most of the new pumps are running within 5% of their best efficiency
points. The pumps were mostly designed correctly into their new systems, and run for
various years without problems, an amazing feat of engineering, math, and art ...
before computers.

Remember that we're calculating the TDH. Two elements of the TDH, the Hs and the
Hp, were determined mostly by observation of the system and drawings. The
remaining two elements, the Hf and the Hv, are the most illusive and difficult to
calculate. Yet, they determine how and where the pump will operate on its curve.
Continue reading.

Using the formulas, the K values, and the pipe schedule tables found in
the Hydraulic Institute Manual, (Vsuction = 3.33 ft/sec for 6 inch pipe @
300 GPM and Vdischargc 7.56 ft/sec for 4 inch pipe Q 300 GPM) or
=
other source, we can estimate or calculate the friction and velocity
heads in the system. Because the Hv is used to calculate the Hf, we'll
begin with the Hv. The formula is:
System Hv = Hv suction + Hv discharge

= V2 i 2g suction + V2 i 2g discharge
= 3.332 + 64.32 + 7.562 + 64.32
= 0.172 ft. suction + 0.888 ft. discharge
System Hv = 1.06 feet
The Hv suction and discharge values will be used in the Hf formula.
System Hf = Hf pipe + Hf elbows + Hf valves + Hf tramp flanges
+ Hf other
Taking this formula in groups, we begin with the Hf pipes.

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